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u                          BODY CAVITY AND DIAPHRAGM                        69
into lobules ; this is the case with the Whales, the Bears, the
Oxen, and a few other forms. A curious fact about the kidneys
of the Mammalia is their very general asymmetry of position.
One of them usually lies in a more advanced position than the
other. The ureters lead from the kidneys to the urinary bladder,
which in its form and relations is quite distinctive of the
Mammalia. The bladder is formed out of the remains of the
allantois, and is therefore not the exact homologue of the bladder
of the frog, which is the equivalent of the entire sac which grows
out of the cloaca in the mammal, and is the foetal allantois.
The ureters open into the bladder in the higher Mammalia., but
lower down in the urino-genital passage in the more primitive
mammals.
The Body Cavity.—The Mammalia differ from all other living
vertebrates by the arrangement of the body cavity in which lie
the viscera. That cavity is divided into two by a partly muscular
and partly tendinous partition, the diaphragm. -No other verte-
brate has this precise disposition of the coelom. The diaphragm
lies usually transversely to the longitudinal axis of the body,
but gets a much more oblique arrangement in the Cetacea and
the Sirenia, whose needs demand a more expanded chamber for
the lungs. For in front of the diaphragm lie the lungs and
heart; behind it the stomach, liver, intestines, and the organs
of reproduction and excretion. The diaphragm is used in re-
spiration ; when its muscles contract, the surface jiirected toward
the pleural cavity becomes less convex, and the cavity of the lungs
is thus increased, allowing them to expand under the pressure of
the entering air.
The Lungs.—The lungs of the Mammalia differ from those
of animals lying lower in the series by the fact, just referred to,
that they occupy a pleural cavity completely shut off from the
abdomen by the diaphragm. As a rule the lungs of the Mammalia
are to be distinguished by their more or less extensive lobation.
In the Whales, however, and in the Sirenia, they are not much
divided, but present the appearance of the simple sac-like lungs
of the reptiles. In some mammals there is a median and
posterior unpaired lobe of the lung, which lies in the post-
pericardial cavity behind the pericardium. This is not uni-
versally present. The lungs are very frequently not symmetrical
in their lobation, the number of separate lobes on the right side